From the prior art is known an optical fiber acting as a slab-coupled waveguide. The cross-section of such a fiber comprises a core, which is two-dimensional and responsible for the horizontal confinement of the fiber's fundamental mode. A slab is placed in the vicinity of the core for acting as a mode sink for the core. The width of the slab is at least three times of the width of the core. A cladding surrounds the core and the slab. The cladding is made of one or several materials with refractive indices lower than the core and slab materials. The core, the slab and the cladding and any other protective or supportive structure attached to them together form an overall structure which determines the mechanical properties of the fiber.
Such rib fibers, as well as slab-coupled waveguides in general, were invented already in the early 1970s as is described in the literature:
1) P. Kaiser et al., “A new optical fiber”, The Bell System Technical Journal, 52, pp. 265-269, 1973,
2) E. A. J. Marcatili, “Slab-coupled waveguides”, The Bell System Technical Journal, 53, pp. 645-674, 1974, and
3) J. A. Arnaud, “Transverse coupling in fiber optics Part II: Coupling to mode sinks”, The Bell System Technical Journal, 53, pp. 675-696, 1974.
Their main advantages were identified already then, namely their insensitivity to wavelength and scaling of dimensions, as well as their ability to provide single-moded operation with high refractive index contrast and large core dimensions. The rib fibers were seen to avoid the problems that were then associated with alternative fiber technologies, especially the difficulty in providing fibers with small and accurate refractive index differences. The proposed and demonstrated rib fibers had a core and slab made of one homogeneous piece of glass, surrounded by an air cladding, and thus called “single-material fibers”. Their sensitivity to bending in the direction of the slab (horizontally) was carefully studied and it was already then recognized that the rib fibers could have a shorter tolerable bending radius in the perpendicular (vertical) direction. However, no detailed analysis of vertical bending was carried out at that time.
During and after the 1970s, alternative fiber structures based on doped glass were developed further and they provided much better results than the “single-material fibers”. The rib fibers were then forgotten, probably because they had higher scattering and bending losses and they were more fragile than the alternative fiber structures. Presently circular symmetric fibers with small refractive index differences based on glass doping clearly dominate the fiber optic market, while the “single-material fibers” and rib fibers are in practice totally forgotten.
Present fiber technology is quite mature, but the continuous increase of channels in wavelength division multiplexing (WDN) and growing interest on high-power fibre lasers has generated a need for fibers with large mode area (LMA) and, thus, reduced nonlinear effects, but only one or few propagating modes. These contradicting requirements are difficult to fulfill with conventional fibers. Coiling with a fixed radius of curvature can be used to eliminate the higher order modes without adding too much bending losses for the fundamental mode. However, it is not very convenient to have a fixed radius of curvature. Therefore, a novel solution is needed to provide fibers with large mode area (i.e. large core), small number of modes (preferably single-moded), small propagation loss for the fundamental mode, and insensitivity to bending.
On planar substrates, such as silicon and compound semiconductor wafers, the rib structure and slab-coupled waveguides in general, have become a very common solution. Their main advantage there is the ability to tailor the effective index difference, confinement and number of modes in a waveguide by the waveguide dimensions, instead of changing the refractive indices of the materials. For example, in silicon waveguides a core with reactangular cross-section should have a thickness and width well below 0.5 μm in order to have single-moded operation. A silicon rib waveguide can be single moded even with the thickness and width well above 10 μm. The waveguiding properties of the rib structure and slab-coupled waveguides in general, are discussed thoroughly in literature, e.g. in:
4) R. A. Soref, J. Schmidtchen, K. Petermann, “Large single-mode rib waveguides in GeSi—Si and Si-on-SiO2”, IEEE J. Quantum Electron. 27, pp. 1971-1974, 1991,
5). Aalto et al., “Fabrication and characterization of waveguide structures on SOI”, Proceedings of SPIE, 4944, pp. 183-194, 2003, and
6) T. Aalto et al., “Development of silicon-on-insulator waveguide technology”, Accepted for publication in Proceedings of SPIE, 5355, 2004.
Based on the experimental work carried out with slab-coupled waveguides on planar substrates, the main disadvantage is their sensitivity to bending, which leads to the use of very long bending radii and poor miniaturization. The fundamental reason for this is the horizontal spreading of the modes into the slab, i.e. the same mechanism that enables the single-moded operation. The effect becomes stronger when the waveguide dimensions increase, thus limiting the waveguide size. In planar technology there is no way to take advantage of the smaller sensitivity to bending in the vertical direction.